The present invention relates to gas-tight tubes made from aluminum oxide ceramic material with an interior coating. Gas-tight ceramic tubes are used in the BMA process at high temperatures to convert a mixture of methane and ammonia catalytically to hydrogen cyanide and hydrogen (compare Chemie-Technik 1978, p. 231). The conversion is carried out on a platinum catalyst which has been applied to the interior of an aluminum oxide tube. The tubes are suspended inside a combustion chamber and are maintained at about 1250.degree. C.
The tubes employed must be gas-tight and resistant to high temperatures. Furthermore, their interior must be capable of taking up the required amount of platinum per unit area. The coating of an Al.sub.2 O.sub.3 catalyst tube with a platinum solution is described in Example 1 of DBP 3,034,957. The process has the shortcoming that in each case only a part of the required platinum may be applied in one drying operation. For this reason the application of the catalytic metal to the interior of the tube requires several drying operations. The manufacture of aluminum oxide tubes which are suitable for the BMA process and which are assigned to the material classes KER 708 and KER 710 according to DIN 40685, has been known for a long time as the state of the art. The object has been to form these tubes in such a manner that the coating process can be carried out in fewer operations.